Sintered electrodes for a cold cathode tube and cold cathode tubes comprising the electrodes have hitherto been used, for example, as backlights for liquid crystal display devices. In addition to high brightness and high efficiency, a long service life has been required of the cold cathode tubes for liquid crystal display devices.
Cold cathode tubes useful as backlights for liquid crystal display devices generally have a construction comprising a glass tube having an inner surface coated with a phosphor, a very small amount of mercury and rare gas filled into the glass tube, and a lead-in wire or a lead rod (for example, a KOV foil+a high-melting point metallic lead-in wire+a dumet wire) mounted at both ends of the glass tube. In these cold cathode tubes, upon the application of voltage across both the ends, the mercury sealed into the glass tube is evaporated to release ultraviolet light which is absorbed by the phosphor to emit light.
A nickel material has hitherto been mainly used as the electrode. In the nickel electrode, however, the cathode drop voltage necessary for releasing electrons from the electrode into the discharge space is somewhat high. In addition, the service life of the lamp is likely to be lowered due to the occurrence of a phenomenon of the so-called sputtering. The sputtering phenomenon is that, during lighting of the cold cathode tube, ions collide with the electrode, and the electrode material scatters resulting in the accumulation of the scattered material, mercury and the like on the wall surface within the glass tube.
The sputtering layer formed by the sputtering phenomenon takes in mercury. Consequently, the mercury no longer can be utilized for light emission. Accordingly, lighting of the cold cathode tube for a long period of time causes an extreme lowering in the brightness of the lamp and reaches the end of the service life. For this reason, if the sputtering phenomenon could be reduced, the mercury consumption could be reduced and, thus, the service life could be prolonged even when the mercury sealing amount is identical.
Accordingly, an attempt to realize a reduction in cathode drop voltage and the suppression of the sputtering has been made. Specifically, an electrode design, in which the electrode has a closed end and is cylindrical to aim at a reduction in cathode drop voltage and sputtering suppression by holocathode effect, has recently been proposed (Japanese Patent Laid-Open No. 176445/2001 (patent document 1)). Further, the adoption, as the electrode material, of Mo (molybdenum), Nb (niobium) or the like, which can lower the cathode drop voltage by about 20 V, instead of nickel which has hitherto been, has also been proposed.
The closed-end cylindrical electrode for a cold cathode tube proposed in patent document 1 is more advantageous in a reduction in cathode drop voltage and the service life than the conventional nickel electrode. The closed-end cylindrical electrode, however, suffers from a problem that, since the closed-end cylindrical shape is formed from a plate material (generally having a thickness of about 0.07 to 0.2 mm) by drawing, the yield of the material is poor, and, for metals having poor drawability, for example, cracking occurs during drawing. Drawing of the plate material is also disadvantageous in that the cost is high.
In order to overcome the above problems, Japanese Patent Laid-Open No. 178875/2004 (patent document 2) proposes a closed-end cylindrical shape using a sinter of Mo or the like.    [Patent document 1] Japanese Patent Laid-Open No. 176445/2001    [Patent document 2] Japanese Patent Laid-Open No. 178875/2004    [Patent document 3] Japanese Patent Laid-Open No. 242927/2003